Stem cells are critical in normal development and understanding their biology is essential for a fuller understanding of tissue senescene and regenerative potential, as well as many types of neoplasia. The long-term goal of this project is to understand the mechanisms by which stem cells are established during embryogenesis, maintained during post-embryonic development, and then recruited to differentiate at particular times and places both in the developing adult and during regeneration. This work exploits the post-embryonic development of zebrafish melanophores, homologues of mammalian melanocytes, as an especially tractable system for studying stem and progenitor cell biology, with the potential to identify common and essential features of stem cell systems more generally. Studies in Aim 1 will test mechanisms required during embryogenesis to establish precursors to post-embryonic melanophores that will differentiate only weeks or months later, with particular emphasis on ErbB signaling and roles played by embryonic melanophore and glial lineages. The experiments proposed in Aim 2 focus on the morphogenetic behaviors and niches of melanophore stem cells and their progeny during later post-embryonic development, and whether genetically independent pools of progenitors contribute to the adult complement of melanophores. Finally, Aim 3 addresses a later phase in this lineage, when stem cells are recruited to differentiate as melanophores, and how the local tissue environment affects the survival and migration of these cells, here dissecting roles for a novel gene, basonuclin-2, likely required for skin development and homeostasis, as well as the expression of trophic factors required by adult melanophores. Together, these studies will answer several critical questions about this post-embryonic, stem-cell dependent lineage, and will be relevant to a variety of other stem cell systems in zebrafish and mammalian development, as well as in human disease.